So if your chat notifications sound is not working, here are some steps to make sure everything is setup properly:

1- Make sure you are using google chrome (as it is only functional on chrome)

2- If you are on chrome and it is not functional, make sur the sound is selected in settings of chat console`

3-If the two first step did not work type on your google chrome address bar: chrome://settings/content and scroll till you see flash and change to allow sites to run flash or ask first before. Try either or depending on chrome version. (see screenshot)